This invention relates to the field of cryogenic testing of material to determine the thermal performance of a material or system of materials.
One valuable technique for testing the thermal performance of materials, preferably insulation material, is boil-off testing. Boil-off testing is accomplished by filling a vessel with a fluid which boils below ambient temperature. Although the preferred fluid is the cryogen liquid nitrogen, other fluids such as liquid helium, liquid methane, liquid hydrogen, or known refrigerants may be used. Once the vessel is filled with the cryogenic fluid, the vessel is surrounded with the testing material. A calorimetry method is then used to determine the thermal conductivity of the testing material by first determining the amount of heat that passes through the test material to the vessel containing cryogenic fluid. The cryogenic fluid boil-off rate from the vessel is directly proportional to the heat leak rate passing through the test material to the cryogenic fluid in the vessel. For a test material under a set vacuum pressure, the apparent thermal conductivity (k-value) is determined by measuring the flow rate of cryogenic boil-off at given warm and cold boundary temperatures across the thickness of the sample.
Although cryogenic boil-off techniques and devices have been prepared to determine the thermal conductivity of insulation material, the previous techniques and devices are undesirable for a variety of reasons. First, few such cryogenic devices are in operation because of their impracticality from an engineering point of view. The previous cryogenic boil-off devices made it extremely difficult to obtain accurate, stable measurements and required extremely long set up times. Prior testing devices also needed highly skilled personnel that could oversee the operation of the cryogen testing device for extended periods of time, over 24 hours to many days in some cases. Additionally, constant attention was required to operate previous cryogenic testing devices to make the necessary fine adjustments required of the testing apparatus. Second, prior cryogenic testing devices contained the limitation that they did not permit the testing of continuously rolled products which are commonly used insulation materials. The testing of high performance materials such as multilayer insulation requires extreme care in fabrication and installation. Inconsistency in wrapping techniques is a dominant source of error and poses a basic problem in the comparison of such materials. Improper treatment of the ends or seams can render a measurement several times worse than predicted. Localized compression effects, sensor installation, and outgassing are further complications. Third, measurements of various testing parameters were not carefully determined or controlled in previous testing devices. Measurement of temperature profiles for insulation material was either not done or was minimal because of the practical difficulties associated with the placement, feed-through, and calibration of the temperature sensors. Vacuum levels were restricted to one or two set points or not actively controlled altogether. Fourth, previous cryogenic testing devices required complex thermal guards having cryogenic fluid filled chambers to reduce unwanted heat leaks (end effects) to a tolerable level. The previous technique for providing thermal guards, filling guard chambers with the cryogen, caused much complexity both in construction and operation of the apparatus. Known techniques add the further complication of heat transfer between the test chamber and the guard chambers due to thermal stratification of the liquid within the chambers
To eliminate or minimize the foregoing and other problems, a new method of fabricating and testing cryogen insulation systems has been developed. In particular, the present invention ad overcomes the foregoing problems by providing a cryogenic testing apparatus having a boil-off calorimeter system for calibrated measurement of the apparent thermal conductivity (k-value) of a testing material, preferably insulation material, at a fixed vacuum level. The cryogenic testing apparatus includes a vacuum chamber that contains an inner vessel that receives cryogenic fluid, for example liquid nitrogen, helium, hydrogen, methane or other known refrigerants. The apparatus incorporates a number of design features that minimize heat leak, except through specific portions of the inner vessel. For example, the top and the bottom of the inner vessel are abutted with thermal guards, such as silica aerogel composite plugs, to ensure thermal stability of the cryogenic environment. The inner vessel with the thermal guards is called a cold mass assembly upon which the test specimen is installed. The heat leak rate through the top and bottom of the inner vessel is reduced to a fraction of the heat leak through the sidewalls of the vessel. Temperature sensors are placed between layers of the testing material that is wrapped around the cold mass assembly to obtain temperature-thickness profiles. The apparent thermal conductivity (k-value) of the testing material is determined by measuring the boil-off flow rate of the cryogenic fluid and temperature differential between a cold boundary temperature and a warm boundary temperature for a known thickness of the testing material.
During the preferred use, the cold mass assembly is easily and quickly removed from the vacuum chamber and placed on an insulation-wrapping machine preferably using special handling tools. Temperature sensors, preferably thermocouples, are placed at various thicknesses within the testing material. A first temperature sensor on the inner vessel is designated the cold boundary temperature sensor. The cold boundary temperature may also be determined from the known boil-off temperature of the cryogenic fluid. A second temperature sensor on the outer surface of the testing material is designated the warm boundary temperature ID sensor. The warm boundary temperature sensor may be placed at any known distance from the inner vessel. After the testing material is secured to the cold mass assembly, the cold mass assembly is installed within the vacuum chamber using a special handling tool such that the insulation test specimen remains undisturbed and untouched. Preferably, the cold mass assembly is suspended by a plurality of support threads, such as three KEVLAR threads with hooks and hardware for attachment and length adjustment. KEVLAR threads have a low thermal conductivity, a high tensile strength and greatly resist elongation. Therefore, a relatively small diameter KEVLAR thread is preferred to minimize any additional heat transfer to the inner vessel. Once the cold mass assembly is secure, the handling tool is removed, and the vacuum chamber is sealed, the cryogenic fluid is supplied to the inner vessel, preferably using a specially designed funnel and fill tube, until the inner vessel is full and at a constant temperature. The vacuum chamber is maintained at a constant vacuum, using a preferred vacuum pumping and gas metering system, and a set sidewall temperature, using a preferred electrical heater system. The temperature differential between the cold boundary temperature and the warm boundary temperature of the testing material is measured by the temperature sensors and these values, along with the boil-off flow rate and the material thickness, are used to compute the comparative k-value. Calibration of the device, that is, determination of the total parasitic heat leak rate or xe2x80x9cend effectsxe2x80x9d, is accomplished by testing a material with a known k-value under the pressure and temperature conditions of interest. The actual k-value will therefore by slightly lower than the comparative k-value.
The present invention will overcome many shortcomings of the previous cryogenic boil-off devices. First, the testing device is more practical from an engineering viewpoint as compared with the previous devices. The present device can be employed in an automatic operation that requires little oversight by the operator. The design of the preferred silica aerogel i stacks as thermal guards is high performance and robust so that heat leak performance does not drift over time, thus resulting in a system calibration having long-term reliability and repeatability. The unique funnel and fill tube design allows for a one step cooling, filling and thermal stabilization process and eliminates the need for separate fill and vent ports in the inner vessel. This single port for filling and venting is constructed, in part, from thin wall stainless steel bellows which greatly increase the length of the path for conduction of heat from the vacuum chamber to the inner vessel. The parasitic heat leak to the inner vessel is therefore reduced to a minimum. Second, the present invention allows for the testing of large size prototype material systems in typical actual-use configurations. Of critical importance to the present invention is the ability to test continuously rolled insulation materials. This is highly desirable because other forms of insulation material, such as seamed blankets, can drastically affect the test results producing inaccurate readings in many cases. Although testing of continuously rolled insulation material is the preferred material to be tested by this device, a variety of other materials, other forms of material, or other components, may also be tested using the device. For example, bulk fill materials are tested using a containment sleeve with low thermal conductivity supports at the top and bottom. Other materials, including rigid or flexible types and clam-shell or blanket forms, are tested by afflixing the test specimen to the outside circumference of the cold mass assembly using tapes, wires, or other suitable means of attachment. Additionally, the ability to quickly change out the test article with another material is accomplished by the present invention. Third, a means for measuring the temperature profile across a known thickness of the insulation material is accomplished in order to characterize and understand the performance of the insulation system. Full range vacuum levels, varying from high vacuum to soft vacuum to atmospheric pressure, to higher pressures can be tested with a single device. Different residual gases such as air, nitrogen, helium, or carbon dioxide can be supplied to the vacuum chamber. The vacuum level can be maintained at a very steady value for long periods of time with accurate vacuum control and measurement. Fourth, the use of the preferred custom designed silica aerogel composite stacks for thermal guards eliminates the need for guard chambers containing cryogenic fluid. This feature also eliminates the problem of the effect of thermal stratification of the liquid inside the test chamber.